D2O trimer: Intermolecular vibration-tunneling states, low-frequency spectrum, and the effects of the three-body...

We present the first rigorous 12D quantum calculations of the intermolecular vibration-tunneling (VT) states of (D2O)3, in which the monomers are treated as rigid. They are performed using the recently developed methodology [Simkó, Felker, and Bačić, J. Chem. Phys. 162, 034301 (2025)], which was utilized in the 12D quantum treatment of (H2O)3. A high-quality ab initio 2 + 3-body water potential is employed. The 12D eigenstates of (D2O)3 are used to simulate the low-frequency absorption spectrum of this trimer. Through the comparison of the results obtained here for (D2O)3 with those computed previously for (H2O)3, we elucidate the effects of the substitution of all H atoms of water trimer with D atoms on the VT states and the low-frequency spectrum. They are substantial particularly for the large-amplitude torsional vibrations, whose energy levels in (D2O)3 are, by a factor of two, lower than those of (H2O)3. The entire low-frequency absorption spectrum of (D2O)3, which is dominated by the bending transitions, is strongly redshifted in comparison with the spectrum of (H2O)3. The deuteration of the intermolecular stretching vibrations lowers their energy by only a few wave numbers, since it does not change much the masses of the water monomers. In addition, calculations are performed for (D2O)3, which directly reveal the significant effects of the three-body interactions on the vibrational energy levels and the low-frequency spectrum of the trimer.